The rigorous conditions of very low operating temperatures coupled with high pressure impose extreme difficulties for controlling cryogen leakage from in-line tube fittings. The KC126 fitting, which has been used by NASA in the fuel lines of the space shuttle, has been found to leak. We have shown that the KC126 fitting leaks at 205K, and at 77K has a leak mass flux of 1.28.times.10.sup.-3 kg/min. See also Moore, Z., D. Capellin, A. Rodriguez, J. England (1988) "LH.sub.2 TSM Leakage Problem," Interim Report: DM-MED-4, NASA, J.F.K. Space Center. Accordingly, there exists a need for an improved design of cryogenic seal for tube fittings such as those used in the space shuttle, and which is reusable and seals under extremes of low temperature and high pressure. Such a seal would have wide application in the cryogenic art.
The problem of sealing a joint between two interconnecting pieces that are designed to operate at cryogenic temperatures has been previously recognized. In part, this problem was addressed in U.S. Pat. No. 3,630,533, issued Dec. 28, 1971 to Butler et al., entitled "Dynamic Seal for Cryogenic Fluids." An additional problem addressed by Butler et al. is the high temperature sealing problem, which influenced Butler et al.'s design. Butler et al. used a circular sealing ring made from a fluorocarbon plastic material to seal two metal tubular couplings. The sealing ring is pressed onto a radially outward surface of one of the metal couplings. The sealing ring has a radially inward protruding annular rib which elastically and inelastically deforms as the sealing ting is pressed into place. This arrangement effects a sealing engagement between the sealing ting and the metal coupling at temperatures reported to be within the range of 70.degree. F. to -423.degree. F. At normal temperatures, the inner surface of the sealing ring is held in sealing engagement by the elastic preload induced by the initial interference press-fit and deformation of the protruding annular rib. In addition to the preload, a circumferential tension is generated in the sealing ring as the temperature decreases, because the sealing ring's coefficient of thermal expansion and contraction is greater than the coefficient of thermal expansion and contraction of the metal couplings. Thus, because of the differences in the expansion coefficients of the sealing ring and the metal coupling that it is pressed onto, the sealing engagement between these members of two different materials becomes tighter as the temperature decreases. However, in Butler et al.'s design, each time two metal tubular couplings are sealed together, the complex plastic sealing ring must be inelastically deformed into a particular configuration. Since the sealing ring is irreparably deformed by its installation, after separation of the two couplings for maintenance or other reasons, it is necessary to replace the sealing ring before the two couplings can be rejoined. Such a "use once and throw away" approach is wasteful, ultimately expensive, and troublesome if a replacement ring is not readily available. A coupling between two members which can be joined and separated, without requiring a new sealing ring each time, is needed.